https://leetcode.com/problems/letter-combinations-of-a-phone-number/

leetcode-17.java

import java.util.ArrayList;

class Solution {
    public List<String> letterCombinations(String digits) {
        ArrayList<String> output = new ArrayList<>();
        String[] arr = getLettersArr(digits);

        if (digits.isEmpty()) return output;
        recur(arr, "", output);
        
        return output;
    }

    private void recur(String[] arr, String cur, ArrayList<String> output) {
        if (arr.length < 1) { output.add(cur); return; }

        for (int i = 0; i < arr[0].length(); i++) {
            recur(shiftArr(arr), cur += arr[0].charAt(i), output);
            cur = cur.substring(0, cur.length() - 1);
        }
    }

    private String[] shiftArr(String[] arr) {
        String[] newArr = new String[arr.length - 1];
        for (int i = 1; i < arr.length; i++) newArr[i - 1] = arr[i];
        return newArr;
    }

    // Convert numbers to String of letter options
    private String[] getLettersArr(String digits) {
        String[] letters = new String[digits.length()];
        for (int i = 0; i < letters.length; i++) letters[i] = getDigitLetters(digits.charAt(i));
        return letters;
    }

    private String getDigitLetters(char num) {
        if (num == '2') return "abc";
        if (num == '3') return "def";
        if (num == '4') return "ghi";
        if (num == '5') return "jkl";
        if (num == '6') return "mno";
        if (num == '7') return "pqrs";
        if (num == '8') return "tuv";
        if (num == '9') return "wxyz";
        return "";
    }
}